1. R. shoulder radiograph, AP projection

The glenohumeral (shoulder) joint lies in an oblique (i.e., non-sagittal) plane with its anterior aspect medially and its posterior aspect laterally (see Image #5 — axial MR of the R. Shoulder). Therefore on an AP projection the head of the humerus is superimposed over the glenoid fossa; this region of superimposition approximates an ellipse in shape.

This image displays the greater tuberosity (tubercle) in profile at the lateral aspect of the humerus because the arm is externally rotated to the extent that a line between the humeral epicondyles lies in the coronal plane (the anatomic position, more or less).

The AP image of the shoulder would show the lesser tuberosity (tubercle) in profile (at the medial aspect of the humerus) if the arm were internally rotated to the extent that a line between the humeral epicondyles were to lie in a sagittal plane.